The Placenta

Outline the functions of the placenta, and determinants of placental blood flow.

The placenta is an organ of maternal and foetal origin which supports the developing foetus.

Physiological Properties

The placenta has three broad functions:

• Interface between foetus and mother for nutrient exchange
• Immunological barrier
• Endocrine

Nutrient and Waste Exchange Functions

The primary purpose of the placenta is diffusion of nutrients and oxygen, and removal of waste.

As with the lung, diffusion is dependent on Fick's Principle, i.e.:

$\dot{V} = {A \times D \times \Delta P \over T }$, where:

• $\dot{V}$ = Flow of substance across the membrane
• $A$ = Area of the membrane
• $D$ = Diffusion constant for the substance, where $D \propto {Lipid \ Solubility \over \sqrt{Molecular \ Weight}}$
  • Molecules < 600 Da in size more readily diffuse down concentration gradients
• $\Delta P$ = Concentration difference across the membrane
  • Maternal placental flow is ~600mL.min^-1 at term - double that of foetal flow - which improves diffusion by increasing the concentration gradient for solutes
• $T$ = Thickness of the membrane

O₂ Diffusion

At the end of pregnancy, PO₂ for foetal blood:

• Entering the placenta via the umbilical artery is 18mmHg (SpO2 45%)
• Leaving the placenta via umbilical vein is 28mmHg (SpO2 70%)

The foetus is able to have adequate delivery of O₂ despite the low PO₂ for four reasons:

• High Cardiac Index
  Increased cardiac output increases DO₂.

• Foetal Hb
  Contains two gamma subunits instead of beta subunits. These prevent the binding of 2,3-DPG, which result in a left-shifted Oxy-Haemoglobin dissociation curve, favouring oxygen loading at a low PaO₂.

• Foetal [Hb] is 50% greater than maternal [Hb]

• The Double Bohr effect:
  The Bohr effect states that an increase in PaCO₂ right-shifts the oxyhaemoglobin dissociation curve. Conversely, the affinity of Hb for O₂ increases in alkalaemia. The double Bohr effect describes this happening in opposite directions in the foetal and maternal circulations, favouring transfer of O₂ to the foetus:
  • In the placenta, foetal CO₂ diffuses into maternal blood down its concentration gradient
    This makes foetal blood relatively alkaline, and maternal blood relatively acidic. Therefore:
    • O₂ unloading of maternal blood is favoured
    • O₂ loading of foetal blood is favoured

CO₂ Diffusion

CO₂ is extremely lipid soluble, and so passes easily across membranes. Foetal PaCO₂ is ~50mmHg, and intervillous PCO₂ is ~37mmHg. CO₂ offloading is favoured in the foetus by:

• A high Foetal [Hb] increases the amount of CO₂ that can be carried as carbaminohaemoglobin
• The Double Haldane effect:
  The Haldane effect states that deoxygenated Hb binds CO₂ with more affinity than oxygenated Hb. The double Haldane effect describes this happening in opposite directions in the maternal and foetal circulations, favouring CO₂ transfer to the mother:
  • As maternal blood releases O₂, this favours maternal loading of CO₂ without an increase in maternal PCO₂ (Haldane effect)
  • The release of CO₂ from the foetal Hb favours O₂ loading, which in turn favours further maternal O₂ release.

Nutrient Diffusion

In late pregnancy, foetal caloric requirements are high (approximately the same as the mother). Facilitated diffusion of glucose via carrier molecules occurs in trophoblasts.

Active transport occurs for amino acids, Ca²⁺, Fe, folate, and vitamins A and C. Other transporters actively remove substances from foetal circulation.

Immunological Function

The placenta is selectively permeable to IgG via pinocytosis, which allows maternal antibodies to provide passive immunity to the foetus.

Endocrine Function

Synthesises:

• βHCG
• hPL
• Oestriol
• Progesterone

Development

The placenta develops simultaneously from foetus and mother:

• From the uterine wall, the mother produces blood sinuses around the trophoblastic cords
  • These in turn send out placental villi

• This creates a sinus of maternal blood invaginated by multiple foetal villi

• Foetal villi are supplied by two umbilical arteries and a single umbilical vein

• Maternal sinuses are filled from the uterine arteries
• The maternal sinuses are supplied by spiral arteries

Properties of the Developing Placenta

• Thick(er) membrane impairs permeability
  Placental membrane permeability is small in early-to-mid pregnancy, reaching maximum at ~34 weeks
• Smaller surface area

Properties of the Mature Placenta

• Thick membrane - improved permeability
• Surface area of 14m²
• Weight of ~500g
• Blood flow of 600mL.min^-1 at term
  Flow is reduced during contractions due to increased uterine pressure and also with α-adrenergic stimulation.

References

1. Hall, JE, and Guyton AC. Guyton and Hall Textbook of Medical Physiology. 11th Edition. Philadelphia, PA: Saunders Elsevier. 2011.
2. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.